The present invention pertains to serial data link transmission techniques between digital switching systems and more particularly to circuitry for recovery of data from unused bit positions of a T1 span.
Digital trunk units perform interface functions between T1 spans and the central processing unit (CPU) of a digital telephone switching system.
Certain bit positions in a T1 span are unused. These bit positions may be used to transmit other data. The problem is to recover this data from the unused bit position synchronously with the flow of the PCM data of a T1 span. A terminal equipment control circuit provides for detecting this data and retransmitting it to the switching system's CPU.
The terminal equipment control circuit must be synchronized with the particular T1 span from which it is receiving data. A digital trunk unit operates a number of T1 spans. Therefore, the terminal equipment control circuit of a common channel interoffice signaling (CCIS) system must be capable of synchronizing with a number of different T1 spans. If each T1 span generates its transmit and receive clocks, there will be a large number of clock circuits and connections required, one for each T1 span, to maintain synchronism between the digital trunk unit and the terminal equipment control circuit.
In order to accurately retrieve the CCIS data from the unused bit positions of the incoming T1 spans, a detection and synchronization scheme is required. Accordingly, it is the object of the present invention to provide for synchronously detecting CCIS data in unused bit positions of a number of T1 spans while minimizing the number of synchronization clock sources.